Changes in fatty acids of leaf polar lipids during chilling and post-chilling rewarming of Zea mays genotypes differing in response to chilling

Changes in fatty acids of leaf polar lipids: monogalactosyldiacylglycerol (MGDG), digalactosyldiacylglycerol (DGDG), sulfoquinosyldiacylglycerol (SQDG) and phosphatidylglycerol (PG) in maize seedlings of chiling-sensitive (CS) CM 7 and Co 151 lines and chilling-tolerant (CT) S 215 and EP 1 lines upon chilling for either 4 or 6 days in the dark and after rewarming for 4 days at original growth conditions were studied. The content of free fatty acids (FFA) in control leaves as well as alterations in the proportion of major fatty acids, unsaturation ratio (UR), double bond index (DBI) and changes in the proportion of heigh-temperature melting of both phosphatidylglycerol (htm-PG) and sulfoquinovosylglycerol (htm-SQDG) after chilling and rewarming of seedlings were estimated. FFA content in intact leaves was 2–3-fold higher in the chilling susceptible CM 7 line than in the other three inbreeds studied. After chilling for 6 days the level of FFA increased only in CM 7 and S 215 lines by about 30 %. Upon rewarming seedlings chilled for 6 days the level of FFA increased about two-fold in CS Co 151 line and CT EP 1 line and decreased in CS CM 7 line. Limited accumulation of FFAs during chilling and post-chilling rewarming of maize seedlings, did not correspond to the extent of polar lipid breakdown (Kaniuga et al. 1999b) probably due to the contribution of active oxidative systems to the peroxidation of fatty acids under these conditions. During rewarming seedlings chilled for 6 days major changes were observed in decrease of 18:3 and an increase of 16:0 in all four polar lipids studied with more pronounced changes in CS than CT lines. Similarly, in CS inbreeds a decrease in UR of fatty acids in MGDG, DGDG and SQDG after post-chilling rewarming was greater than in CT lines. Proportion of htm-fraction in both PG and SQDG increased after post-chilling rewarming in all four inbreeds, however to a lesser extent in CT than CS lines. A similar pattern of changes in DBI in CS and CT maize seedlings was observed in glycolipid and combine lipid classes. More extensive degradation of polar lipids in CS than CT maize inbreeds following galactolipase action in chloroplasts (Kaniuga et al. 1998) provides FFAs for initiation of peroxidation by LOX which is manifested by decrease of UR and DBI. This sequence of reactions during chilling and post-chilling rewarming appears to be a main route of fatty acids peroxidation responsible for secondary events involved in chilling injury. In addition, the extent of these changes differentiates CS and CT inbreeds. Contribution of esterified fatty acids in thylakoid lipids to direct peroxidation, may be of minor importance.     

Degradation of leaf polar lipids during chilling and post-chilling rewarming of Zea mays genotypes reflects differences in their response to chilling stress. The role of galactolipase

Degradation of leaf polar lipids [monogalactosyldiacylglycerol (MGDG), digalactosyldiacylglycerol (DGDG), sulfoquinovosyldiacylglycerol (SQDG) and phosphatidylglycerol (PG)] and chlorophyll (Chl) were studied in four Zea mays genotypes differing in chilling susceptibility following dark chilling and post-chilling rewarming at original growth conditions. Assessment of visual chilling injury symptoms during post-chilling rewarming differentiated maize inbred lines into chiling-sensitive (CS) CM7 and Co151 lines and chillingtolerant (CT) S215 and EP1 lines. Severity of chilling injury in CS and CT inbreeds were correlated with the extent of Chl and polar lipids degradation. Chilling for either 4 or 6 days followed by 4 days of rewarming caused more extensive degradation of total polar lipids content in CS than in CT lines. MGDG decreased mostly during chilling whereas DGDG dropped during rewarming only. Chl content was not affected during chilling but its large decrease, greater in CS than in CT lines, was observed upon rewarming. Extent of polar lipids breakdown in CS and CT inbreeds during chilling and post-chilling rewarming is correlated with galactolipase activity in chloroplasts (Kaniuga et al., 1998) and visual assessment of chilling injury. In view of the data it is likely that contribution of galactolipase activity induced during low-temperature stress of CS plants is an important factor responsible for thylakoid lipid degradation and development of chilling injury as postulated previously (Kaniuga 1997). It is suggested that genetically engineered reduction of galactolipase activity or elimination of the factors(s) involved in induction/stimulation of its activity during chilling might increase tolerance of CS species to chilling stress.     

Comparison of galactolipase activity and free fatty acid levels in chloroplasts of chill-sensitive and chill-resistant plants

Galactolipase activity in chloroplasts of several chill-resistant plants was found to be very low [0.02-0.13 mumol free fatty acid (FFA) liberated min-1 mg protein-1] or not detected. The same phenomenon was observed for soybean and members of the Cucurbitaceae such as cucumber, pumpkin, melon and squash. Since, following cold storage of cucumber leaves, the levels of monogalactosyl-diacylglycerol and digalactosyl-diacylglycerol in chloroplasts decrease while those of FFA accumulate it seems likely that in these typical chill-sensitive plants galactolipase is present but inactivated during isolation procedure. The low galactolipase activity in chloroplasts was accompanied by a relatively low FFA content ranging from 0.05 mumol to 0.30 mumol FFA mg chlorophyll (Chl)-1. However, both pea and horse bean chloroplasts (with low galactolipase activity) exhibit about 0.45 mumol FFA mg Chl-1. Elevated galactolipase activity was observed in chloroplasts of most chill-sensitive species (ranging from 0.31 mumol to 1.32 mumol FFA liberated min-1 mg protein-1) as well as in chloroplasts from broad bean, a member of a chill-resistant species (1.26 mumol FFA liberated min-1 mg protein-1). In addition in this latter group of plants FFA level in chloroplasts often did not fit the galactolipase activity. The results suggest that there exists a tendency for chilling tolerance of plants to decrease both galactolipase activity and FFA level. However, in some plant species with elevated galactolipase activity the chloroplast FFA level does not correlate with enzyme activity.     

Galactolipase activity and free fatty acid levels in chloroplasts of domestic and wild tomatoes with different chilling tolerance

In order to establish differences in the chilling sensitivity of domestic and wild Lycopersicon species, galactolipase (EC 3.1.1.26) activity, free fatty acid (FFA) level and Hill reaction activity were measured in chloroplasts isolated from control and cold treated leaves of L. esculentum Mill., cv. Norton, L. hirsutum Humb. and Bonpl., L. peruvianum var. glandulosum Mill. Galactolipase activity was higher in chloroplasts from Lycopersicon species with high chilling sensitivity than in chloroplasts of more chilling-resistant ones. A similar relationship was observed for FFA level in chloroplasts from both cold-stored and control leaves. Decrease in Hill reaction activity due to cold stress was greater in chloroplasts of more chilling-sensitive species. The changes are accompanied by a decline of photochemical activity. Considering the changes in the three parameters noted above, an increasing order of chilling tolerance was established: L. esculentum < L. hirsutum (700 m) < L. hirsutum (3100 m) < L. peruvianum (3400 m). It is suggested that measurements of galactolipase activity and FFA may be useful in an evaluation of differences in resistance to chilling injury of closely related species.     

Galactolipase activity but not the level of high-melting-point phosphatidylglycerol is related to chilling tolerance in differentially sensitive Zea mays inbred lines

Galactolipase activity, the level of high-melting-point phosphatidylglycerol (HMP-PG) as well as degradation of lipids during chilling and rewarming were studied in seedlings of maize inbred lines with different chilling responses. In aged chloroplasts of chilling-sensitive (CS) lines, galactolipase activity was considerably higher than that determined in aged chloroplasts isolated from chilling-tolerant (CT) ones. Chilling of seedlings at 5 °C for 6 days induced neither loss of chlorophyll content nor visible changes in the leaves, while a slight decline in total acyl lipid content by about 15.5% and 12.5% in CS and CT lines, respectively, was observed. Among total acyl lipids, only monogalactosyldiacylglycerol (MGDG) levels were decreased significantly upon chilling. Following return to the original growth conditions for 4 days, visible chilling injury in seedlings as well as essential differences in the decrease in total acyl lipids by about 53% and 20% in CS and CT lines, respectively, were found. These changes were accompanied by more extensive degradation of MGDG, digalactosyldiacylglycerol and phosphatidylglycerol in CS than in CT lines. As the levels of HMP-PG in fresh leaves were the same in all four lines of maize, it seems that galactolipase activity and not the level of HMP-PG is related to chilling response in maize. Received: 4 July 1997 / Revision received: 18 September 1997 / Accepted: 3 March 1998     

Chilling-Enhanced Photooxidation : The Peroxidative Destruction of Lipids during Chilling Injury to Photosynthesis and Ultrastructure

Chilling-induced photooxidation was studied in detached leaves of chilling-sensitive (CS) cucumber (Cucumis sativus L.) and chilling resistant (CR) pea (Pisum sativum L.). The rates of photosynthesis and respiration, measured as O₂ exchange, were found to be comparable in the two species over a temperature range of 5 to 35°C. Chilling at 5°C for 12 hours in high light (1000 microeinsteins per square meter per second) decreased CO₂ uptake 75% in detached pea leaves whereas CO₂ uptake by cucumber was reduced to zero within 2 hours. Respiration was unaffected in either species by the chilling and light treatment. Although ultrastructural alterations were apparent in chloroplasts of both species, cucumber's were affected sooner and more severely. The mechanism of photooxidative lipid peroxidation was investigated by following the production of ethane gas under a variety of conditions. Maximum ethane production occurred in the CS cucumber at low temperature (5°C) and high light (1000 microeinsteins per square meter per second). Atrazine, an inhibitor of photosynthetic electron transport, almost completely halted this chilling- and light-induced ethane production. These data, taken with those reported in an accompanying article (RR Wise, AW Naylor 1986 Plant Physiol 83: 278-282) suggest that the superoxide anion radical is generated in cucumber chloroplasts (probably via a Mehler-type reaction) during chilling-enhanced photooxidation. Parallel experiments were conducted on pea, a CR species. Detached pea leaves could only be made to generate ethane in the cold and light if they were pretreated with the herbicide parquat, a known effector of O₂⁻ production. Even so, pea showed no lipid peroxidation for 6 hours, at which time ethane production began and was at a rate equal to that for the chilled and irradiated cucumber leaves. The results indicate that pea has an endogenous mechanism(s) for the removal of toxic oxygen species prior to lipid peroxidation. This mechanism breaks down in pea after 6 hours in the cold, light, and the presence of paraquat.     

Nitric oxide improves chilling tolerance of maize by affecting apoplastic antioxidative enzymes in leaves

The effects of nitric oxide (NO) on chilling tolerance (freezing injury, ice nucleation activity, contents of hydrogen peroxide and superoxide anion, and lipid peroxidation level) and the activities of apoplastic antioxidant enzymes (peroxidase and superoxide dismutase) were investigated in the leaves of maize (Zea mays) exposed to short-term chilling. NO treatment was carried out through spraying of sodium nitroprusside (SNP), which is a donor of NO, in concentrations of 0.0, 0.1 and 1 μM on the leaves of 10-day plants. The plants then were transferred into the chilling condition (10/7 °C) 2 days before the harvesting of leaves (14th and 21th days). Application of 0.1 μM NO had more effect on the alleviation by decreasing the freezing injury in maize at least for 11 days after the application. Both concentrations of NO generally increased ice nucleation activity of apoplastic proteins extracted from leaves. The SNP applications decreased the contents of reactive oxygen species such as hydrogen peroxide and superoxide anion and the level of lipid peroxidation, while further increasing the activities of the apoplastic antioxidant enzymes studied. The results show that exogenous NO treatment provides important contributions to increasing the chilling tolerance of maize by regulating the biochemical mechanisms of chilling response, including apoplastic antioxidant enzymes. It can be seen that the NO treatment can play positive roles in alleviating chilling-induced damage in maize. Therefore, it is suggested that NO treatments may contribute to research studies related to diminishing chilling-induced damage in agricultural applications.     

Enhanced susceptibility of photosynthesis to low-temperature photoinhibition due to interruption of chill-induced increase of S-adenosylmethionine decarboxylase activity in leaves of spinach (Spinacia oleracea L.)

The possible involvement of polyamines in the chilling tolerance of spinach (Spinacia oleracea L.) was investigated focusing on photosynthesis. During chilling at 8/5C (day/night) for 6 d, S-adenosylmethionine decarboxylase (SAMDC) activity increased significantly in leaves in parallel with the increase in putrescine and spermidine (Spd) content in leaves and chloroplasts. Treatment of leaves with methylglyoxal-bis(guanylhydrazone) (MGBG), an SAMDC inhibitor, resulted in the deterioration of plant growth and photosynthesis under chilling conditions, which was reversed by the concomitant treatment with Spd through the roots. Plants treated with MGBG showed lower photochemical efficiency of PSII than either the control or plants treated with MGBG plus Spd during chilling and even after transfer to warm conditions, suggesting an increase of photoinhibition due to low Spd in chloroplasts. Indeed, MGBG-treated plants had much lower activities of thylakoid electron transport and enzymes in carbon metabolism as well as higher degrees of lipid peroxidation of thylakoid membranes compared to the control. These results indicate that the enhanced activity of SAMDC with a consequential rise of Spd in chloroplasts is crucial for the cold acclimation of the photosynthetic apparatus in spinach leaves.     

Desiccation-tolerant Sporobolus stapfianus: lipid composition and cellular ultrastructure during dehydration and rehydration

The desiccation-tolerant plant Sporobolus stapfianus was subjectedto slow dehydration and to rehydration either as a silica gel-drieddetached leaf or as an airdried plant. In detached leaves dehydrationresulted in a lower relative water content in comparison withleaves dried on the plant. Water loss caused a reduction inchlorophyll, carotenoid and lipid contents and an increase inconjugated dienes. In detached leaves, ultrastructure was alsoaffected by dehydration, showing damaged cells with alteredchloroplasts which retained large quantities of starch and lipid-likeinclusions in the stroma. Upon rehydration a continuous degradationof the chemical composition and cell organization was observedwith a further increase in peroxidation. Leaves dehydrated onthe plant showed degradation of chlorophyll and lipids, whereascarotenoids increased and conjugated dienes decreased. Desiccationcaused a vacuolar fragmentation and a decline in starch, whereaschloroplasts underwent slight alterations. Following rewateringa full recovery of chlorophyll and lipids occurred, while carotenoidsand dienes remained constant. Starch increased in the chloroplastsand there was complete recovery of the ordered cell arrangementand chloroplast organization. Of the chloroplast polar lipids,in both sets of leaves desiccation caused a reduction only inmonogalactosyldiacylglycerol, while phospholipids showed anopposite pattern, increasing in air-dried leaves and decreasingin detached leaves. Rewatering of leaves desiccated on the plantled to a complete recovery of the lipid composition, whereasdetached leaves suffered a complete lipid degradation with theloss of polyunsaturated fatty acids. Key words: Desiccation tolerance, lipids, resurrection plants, Sporobolus stapfianus, ultrastructure     

Alteration in the metabolism of free radicals in wheat seedlings grown in the presence of 4-chloro-5-dimethylamino-2-phenyl-3(2H) pyridazinone

When wheat seedlings were grown in the presence of 62.5-500μM 4 chloro-5-dimethylamino-2-phenyl-3(2H) pyridazinone, an inhibitor of photosystem II electron transport, there was a marked inhibition of in vivo photosystem II electron transport as revealed by the analysis of fast chlorophyll a fluorescence transients in intact leaves and by the inhibition (95% at 500μM) of net photosynthesis in intact leaves Accompanying this inhibition of photosystem II electron transport, there was a decrease in the content of photosynthetic pigments. The extent of lipid peroxidation, measured in terms of malondialdehyde content was not increased; rather it was found decreased. An analysis of in vitro lipid peroxidation of the thylakoid membranes of control and 4-chloro-5-dimethylamino-2-phenyl-3(2H) pyridazinone treated plants in the presence of a sensitizer dye (toluidine blue) showed a similar rate both in the control and treated samples suggesting that the availability of unsaturated fatty acids as a substrate for lipid peroxidation was not limiting even though it decreased in the treated plants. On the other hand, it appears that the availability of the free radicals for lipid peroxidation was decreased byenhanced activity of the enzyme systems involved in the metabolism of free radicals. Measurements of the activities of enzymes involved in the metabolism of free radicals showed an increase in the activities of NADPH-glutathione reductase (6–8 fold) and catalase (15–30%) and a decrease in the activity of superoxide dismutase (30–45%) in the treated plants.     

Tocotrienols, the unsaturated forms of vitamin E, can function as antioxidants and lipid protectors in tobacco leaves

Vitamin E is a generic term for a group of lipid-soluble antioxidant compounds, the tocopherols and tocotrienols. While tocotrienols are considered as important vitamin E components in humans, with functions in health and disease, the protective functions of tocotrienols have never been investigated in plants, contrary to tocopherols. We took advantage of the strong accumulation of tocotrienols in leaves of double transgenic tobacco (Nicotiana tabacum) plants that coexpressed the yeast (Saccharomyces cerevisiae) prephenate dehydrogenase gene (PDH) and the Arabidopsis (Arabidopsis thaliana) hydroxyphenylpyruvate dioxygenase gene (HPPD) to study the antioxidant function of those compounds in vivo. In young leaves of wild-type and transgenic tobacco plants, the majority of vitamin E was stored in thylakoid membranes, while plastoglobules contained mainly delta-tocopherol, a very minor component of vitamin E in tobacco. However, the vitamin E composition of plastoglobules was observed to change substantially during leaf aging, with alpha-tocopherol becoming the major form. Tocotrienol accumulation in young transgenic HPPD-PDH leaves occurred without any significant perturbation of photosynthetic electron transport. Tocotrienols noticeably reinforced the tolerance of HPPD-PDH leaves to high light stress at chilling temperature, with photosystem II photoinhibition and lipid peroxidation being maintained at low levels relative to wild-type leaves. Very young leaves of wild-type tobacco plants turned yellow during chilling stress, because of the strongly reduced levels of chlorophylls and carotenoids, and this phenomenon was attenuated in transgenic HPPD-PDH plants. While sugars accumulated similarly in young wild-type and HPPD-PDH leaves exposed to chilling stress in high light, a substantial decrease in tocotrienols was observed in the transgenic leaves only, suggesting vitamin E consumption during oxygen radical scavenging. Our results demonstrate that tocotrienols can function in vivo as efficient antioxidants protecting membrane lipids from peroxidation.     

Alteration of oxidative and carbohydrate metabolism under abiotic stress in two rice (Oryza sativa L.) genotypes contrasting in chilling tolerance

Abiotic stress is a major limiting factor in crop production. Physiological comparisons between contrasting abiotic stress-tolerant genotypes will improve understanding of stress-tolerant mechanisms. Rice seedlings (S3 stage) of a chilling-tolerant (CT) genotype (CT6748-8-CA-17) and a chilling-sensitive (CS) genotype (INIAP12) were subjected to abiotic stresses including chilling (13/12 degrees C), salt (100mM NaCl), and osmotic (200mM mannitol). Measures of physiological response to the stresses included changes in stress-related sugars, oxidative products and protective enzymes, parameters that could be used as possible markers for selection of improved tolerant varieties. Metabolite analyses showed that the two genotypes responded differently to different stresses. Genotype survival under chilling-stress was as expected, however, CT was more sensitive to salt stress than the CS genotype. The CT genotype was able to maintain membrane integrity better than CS, perhaps by reduction of lipid peroxidation via increased levels of antioxidant enzymes during chilling stress. This genotype accumulated sugars in response to stress, but the accumulation was usually less than in the CS genotype. Chill-stressed CT accumulated galactose and raffinose whereas these saccharides declined in CS. On the other hand, the tolerance mechanism in the more salt- and water-deficit-tolerant CS may be associated with accumulation of osmoprotectants such as glucose, trehalose and mannitol.     

Photosynthetic apparatus of chilling-sensitive plants. XI. Reversibility by light of cold- and dark-induced inactivation of cyanide-sensitive superoxide dismutase activity in tomato leaf chloroplasts

(1) The inactivation of cyanide-sensitive, copper- and zinc-containing superoxide dismutase activity in chloroplasts following cold and dark storage of both detached leaves and growing tomato plants is accompanied by a decrease in copper and zinc content in both chloroplast preparations and butanol extracts of the enzyme. In contrast, this treatment of chloroplast preparations affects neither superoxide dismutase activity nor copper and zinc content. (2) Copper- and zinc-containing superoxide dismutase is not reactivated following the 2–3 h illumination of cold- and dark-stored detached leaves. However, prolonged illumination of growing seedlings results in the restoration of both the enzyme activity and copper and zinc content in chloroplasts. (3) The data suggest that the dissociation of copper, and probably of zinc, from the enzyme during cold and dark treatment of either detached leaves or growing plants and reincorporation of the metals following the illumination of intact plants are responsible for the reversible inactivation of chloroplast cyanide-sensitive superoxide dismutase of chilling-sensitive plants.     

Contrasting effect of dark-chilling on chloroplast structure and arrangement of chlorophyll-protein complexes in pea and tomato: plants with a different susceptibility to non-freezing temperature

The effect of dark-chilling and subsequent photoactivation on chloroplast structure and arrangements of chlorophyll–protein complexes in thylakoid membranes was studied in chilling-tolerant (CT) pea and in chilling-sensitive (CS) tomato. Dark-chilling did not influence chlorophyll content and Chl a/b ratio in thylakoids of both species. A decline of Chl a fluorescence intensity and an increase of the ratio of fluorescence intensities of PSI and PSII at 120 K was observed after dark-chilling in thylakoids isolated from tomato, but not from pea leaves. Chilling of pea leaves induced an increase of the relative contribution of LHCII and PSII fluorescence. A substantial decrease of the LHCII/PSII fluorescence accompanied by an increase of that from LHCI/PSI was observed in thylakoids from chilled tomato leaves; both were attenuated by photoactivation. Chlorophyll fluorescence of bright grana discs in chloroplasts from dark-chilled leaves, detected by confocal laser scanning microscopy, was more condensed in pea but significantly dispersed in tomato, compared with control samples. The chloroplast images from transmission-electron microscopy revealed that dark-chilling induced an increase of the degree of grana stacking only in pea chloroplasts. Analyses of O-J-D-I-P fluorescence induction curves in leaves of CS tomato before and after recovery from chilling indicate changes in electron transport rates at acceptor- and donor side of PS II and an increase in antenna size. In CT pea leaves these effects were absent, except for a small but irreversible effect on PSII activity and antenna size. Thus, the differences in chloroplast structure between CS and CT plants, induced by dark-chilling are a consequence of different thylakoid supercomplexes rearrangements. Dedicated to Prof. Zbigniew Kaniuga on the 25th anniversary of his initiation of studies on chilling-induced stress in plants.     

The elicitor-induced oxidative processes in leaves of Solanum species with differential polygenic resistance to Phytophthora infestans

Previous studies have shown that suspension-cultured cells of Solanum genotypes with various polygenic resistances to Phytophthora infestans differed in activities of early oxidative processes in response to culture filtrate (CF) from this pathogen. These studies have now been extended by analysing production of reactive oxygen species (ROS), lipid peroxidation and Lipoxygenase (LOX, E.C.1.13.11.12) activity induced by CF in detached leaves of S. tuberosum cv Bzura and clone H-8105, polygenically resistant and susceptible, respectively, as well as S. nigrum, nonhost, completely resistant. The relative increase in the ROS production was higher in the susceptible clone H-8105 than in both resistant genotypes. Lipid peroxidation increased only in the nonhost S. nigrum. An increase in lipid peroxidation in S. nigrum leaves coincided with enhanced LOX activity. In both S. tuberosum genotypes, significant increases in LOX activity were delayed and unaccompanied by changes in the level of lipid peroxidation. LOX activity attained a higher level in both of the resistant genotypes than in the susceptible one. The present results suggest that the involvement of both ROS production and LOX activity in the defense strategy in Solanum species/P. infestans interactions.     

The intensity of lipid peroxidation and enzymatic antioxidative activity in potato leaves under action of drought and polystimulin K

The influence long-term soil drought and potato plants treatment by synthetic analog of cytokinin--polystimulin K on intensity of lipid peroxidation processes and enzymatic antioxidative activity have been investigated. It has been found, that the drought induced the shift of prooxidative-antioxidative balance in respect of lipid peroxidation activation in the potato leaves. It was accompanied by the increase of the ethylene output, membrane permeability, as well as decrease of the lipids content and increase in the enzymatic antioxidative activity (catalase and peroxidase). It is shown, that the intensity of peroxidation processes was higher in budding phases, while enzymatic antioxidative activity was higher in flowering phases in potato plants. Plant exogenous treatment by polystimulin K induced both the decrease in peroxidate oxidation processes, stabilization of catalase and peroxidase activity, as well as the increase in potato resistance to drought.     

Reversal by light of deleterious effects of chilling on oxygen evolution, manganese and free fatty acid content in tomato thylakoids is not accompanied by restoration of the original membrane conformation

Free fatty acids (FFA) generated in thylakoids upon chilling of tomato leaves at 0°C for a few days result in release of functionally active Mn and inactivation of O₂ evolution. Chilling does not lead to a decrease in the extrinsic 16, 23 and 33 kDa polypeptides. Upon illumination of chilled leaves both Mn content and O₂ evolution in thylakoids are restored and FFA content is reduced to the level of the control. Photoactivation of O₂ evolution in chilled leaves does not change the ratio of unsaturated/saturated FFA. Constant Arrhenius activation energy (Ea) for O₂ evolution by thylakoids isolated from control leaves was found, whereas it increased at temperatures below 8.0 and 10.5°C in thylakoids from cold-treated and photoactivated leaves, respectively. This indicates that restoration of O₂ evolution as well as of FFA and Mn contents is not accompanied by a complete reversal of membrance conformation.     

Changes in ammonium ion content and glutamine synthetase activity in rice leaves caused by excess cadmium are a consequence of oxidative damage

Ammonium ion accumulation and the decrease in glutamine synthetase (GS)activity induced by CdCl₂ were investigated in relation to lipidperoxidation in detached rice leaves. CdCl₂ was effective inincreasing ammonium ion content, decreasing GS activity and increasing lipidperoxidation. Free radical scavengers (glutathione, thiourea, sodium benzoate)and an iron chelator (2,2-bipyridine) were able to inhibit the decreasein GS activity and ammonium ion accumulation caused by CdCl₂ and atthe same time inhibit CdCl₂-induced lipid peroxidation. Paraquat,which is known to produce oxygen radicals, decreased GS activity, increasedammonium ion content, and increased lipid peroxidation. GS1 appears to be thepredominant isoform present. Excess Cd caused a decrease in GS1 but not in GS2in detached rice leaves. An increase in lipid peroxidation preceded ammoniumionaccumulation and the decrease in GS1 activity. These results suggest that thedecrease in GS activity and the accumulation of ammonium ions in detached riceleaves are a consequence of oxidative damage caused by excess Cd.     

Relationship between CO2 Assimilation, Photosynthetic Electron Transport, and Active O2 Metabolism in Leaves of Maize in the Field during Periods of Low Temperature

Measurements of the quantum efficiencies of photosynthetic electron transport through photosystem II (_PSII) and CO₂ assimilation (_CO2) were made simultaneously on leaves of maize (Zea mays) crops in the United Kingdom during the early growing season, when chilling conditions were experienced. The activities of a range of enzymes involved with scavenging active O₂ species and the levels of key antioxidants were also measured. When leaves were exposed to low temperatures during development, the ratio of _PSII/_CO2 was elevated, indicating the operation of an alternative sink to CO₂ for photosynthetic reducing equivalents. The activities of ascorbate peroxidase, monodehydroascorbate reductase, dehydroascorbate reductase, glutathione reductase, and superoxide dismutase and the levels of ascorbate and α-tocopherol were also elevated during chilling periods. This supports the hypothesis that the relative flux of photosynthetic reducing equivalents to O₂ via the Mehler reaction is higher when leaves develop under chilling conditions. Lipoxygenase activity and lipid peroxidation were also increased during low temperatures, suggesting that lipoxygenase-mediated peroxidation of membrane lipids contributes to the oxidative damage occurring in chill-stressed leaves.     

Lipids of chill-sensitive and -resistant Passiflora species: Fatty acid composition and temperature dependence of spin label motion

Polar lipids were extracted from the leaves of Passiflora species which varied in their resistance to chilling injury. The fatty acid compositions of the 8 major polar lipid classes from P. caerulea (chill-resistant) were generally similar to those of the corresponding lipids from P. flavicarpa (chill-sensitive). Using ESR spectroscopy, the motion of spin-labelled molecules was measured in phospholipids isolated from a range of Passiflora species. The temperature dependence of the motion of the spin labels showed a change at 1° for lipids of the most chill-resistant species and at 9° for the lipids of the most chill-sensitive species. Lipids from other species showed changes at intermediate temperatures, and the greater the chilling sensitivity of the species, the higher was the temperature of the change. It is concluded that pronounced differences in chilling sensitivity of the Passiflora species are correlated with physical differences in their membrane lipids; however, the degree of unsaturation of the lipids is not a reliable guide to chilling sensitivity.